Light responsive time indicators have been in use since early history. An early example of such time indicators is the sundial. The typical sundial includes a flat base and an upright gnomon. The base is marked with angularly disposed indicia. When the apparatus is correctly positioned in relation to the sun, a shadow of the gnomon will fall across a time calibration.
Various other forms of "sundials" typically involve ancient principles empolying the gnomon and its shadow to indicate time against calibrated indicia. Others have made direct use of sunlight by screening the light around slots or through pin holes. One such device is shown in U.S. Pat. No. 4,255,864 to Glendinning. This patent discloses a sun clock in which the top of a hollow box is provided with a light receiving slit. Sunlight is received through the slit and projects onto a calibrated shaded area of the box below. The beam of sunlight is used to indicate the time.
A "pin hole" sundial is exemplified by U.S. Pat. No. 89,585 to J. Johnson. This device makes use of a hollow sphere with a pin hole opening on one side. Sunlight received through the opening projects against the backside of a graduated surface on the sphere. A dot of light appears on this surface in a position related to time calibration to indicate both hourly time and month.
A combination slit and gnomon sundial arrangement is exemplified in 1905 U.S. Pat. No. 783,245 to S. M. Clarke. This device makes use of a hollow triangular box with a slit formed on either side of a gnomon. The slits and gnomon are applied on an angular surface of the box that is positioned to face the sunlight. A translucent surface marked with hourly indicia is on an opposite of the box. The sun's rays are received through the slit and are blocked by the gnomon so bright areas on opposed sides of the gnomon shadow appear on the translucent surface adjacent the time indicia.
U.S. Pat. No. 4,782,472 to Hines was granted in 1988 for a solar clock with digital time display. A cylindrical light gathering tube with slots receive sunlight. The slots are angularly positioned so that direct light is received only at specified times through a day. The sun rays received through the openings are projected against the opposite inside surface of the tube where receptor ends of "fiber optic" filaments are attached. Light other than the direct light coming from the sun through a selected slot will be diffused at areas other than the specific position at which the fiber optic end is situated. Various strands of the fiber optic are connected to a "digital" display. Intensified light, occurring when particular slots are in direct alignment with the sun, show up as bright images on the digital display, arranged to visually represent the proper time.
Many other forms of light screening and reflecting apparatus have been used for time indicators. However, the difficulty with such arrangements is that the light, contrary to many of the examples shown, diffuses through a pin hole into various shapes according to the current angular relation of the apparatus to the sun. The shape of a "dot" or "line" projected through an opening also depends upon the proximity of the surface against which the ray or shadow projects. Thus, there is inconsistency and, hence, inaccuracy inherent in these indicators.
The above problem has been partially resolved to by light refractive time indicators in which light is concentrated through a lens and is focused outside the lens on a particular calibrated surface. For example, U.S. Pat. No. 1,674,161 to Bogory discloses a time measuring device in which a transparent sphere is used to focus sunlight on a hollow sphere of "metallic screen or some other semi-transparent or transparent material". The hollow sphere is concentric with the center of the transparent sphere and set at a radius such that the inner surface of the hollow sphere is at the focus of the transparent sphere. Thus, light is concentrated through the transparent sphere to form a bright dot on the inner surface of the hollow sphere. A third outward sphere is provided with indicia that is selectively visually aligned with the dot on the internal sphere to indicate time and season or month. This device, while apparently very effective, is also quite complicated and expensive to produce.
A somewhat simpler example of a light responsive "clock" is shown in the 1958 U.S. Pat. No. 2,846,768 to Putnam. This patent discloses a "sundial" in which the sun's rays are received through a substantially cylindrical lens. The sun's rays are focused through the lens onto a transverse flat plate that encircles the cylindrical lens. The plate is formed of translucent material and is marked with radial calibrations to indicate the time. The sun's rays are refracted through the cylindrical lens and appear on the surface of the disk as a wedge of light, the apex of which is associated with a current time marking. This device functions well and is substantially simpler and easier to manufacture than the spherical type indicator.
Another transparent block refractive type indicator is exemplified in U.S. Pat. No. 4,373,270 to Russell M. Ousley. This patent discloses a light transmissive sundial formed of a semicylindrical transparent block. A base of the semicylindrical body is angled so a surface of the block faces angularly upward to face the sun. Spaced transparent surfaces are arranged axially along the semicylindrical surface of the body and are spaced apart 15.degree. from one another in relation to the axis of the body. The axial surfaces are flat in relation to the central axis of the dial. This is done evidently to minimize refraction through the solid transparent material. Concave recesses or flutes between adjacent flat transparent surfaces are darkened to eliminate passage of sunlight between the various flat transparent surfaces.
The opposite side of the transparent body is planar, with an axial notch or groove formed along the apparent central axis for the opposed semicylindrical surface. The notch is used as a reference. The bottom surface of the body is marked with time indicating characters. These characters are visible axially from the top end of the body. The viewer must position the device on a flat surface, orient the semicylindrical surface toward the sun, and compare various rays of sunlight that become visible when looking down through the device. The correct time is discerned by judging which of the light rays appears closest to the reference notch formed at the apparent center of the semicylindrical surface.
Manufacture of the Ousley device is quite complex. The alternating transparent flat surface and concave flutes are quite difficult to produce in actual practice. Still further, the user may experience some difficulty in judging which of the rays appears to be closest to the central slot and then tracing the ray back to the appropriate time indicating character.
A need has therefore remained for a light responsive time indicator that is both simple to manufacture and easy for the operator to set up and read.